1. Field of the Invention
The present invention relates to telephone systems, and is particularly concerned with a telephone switching network for digital speech transmission.
2. Description of the Prior Art
The transmission of digitized speech signals in telephone networks is gaining increasing significance, both because of the advantages connected therewith for switching technology and for transmission technology per se.
In previously-known telephone networks for digital speech transmission, as well as in analog networks, connections between two respective subscribers are generally interconnected for the entire duration of a call. Given increasing density of such networks or, respectively, given increasing numbers of subscribers, the difficulties with respect to the number of available channels increase.
It is known that a voice connection for an individual speech is usually exploited over time only to a slight degree, since, on the one hand, the call partners usually speak alternately and, on the other hand, the verbal flow of the individual partner does not proceed continuously. Even in a short monologue, speech pauses respectively occur during which the appertaining voice connection is, indeed, not required. This fact, as is known, is advantageously exploited by means of methods for saving channel capacity, particularly in cost-intensive connection segments, for example, transatlantic cables, satellite connections and the like. The TASI (time assigned speech interpolation) method represents a method which is known and proven in this respect.
In the speech interpolation which is undertaken in the known TASI method, a respective channel is through-connected for an appertaining voice connection when there is actual speaking in the appertaining direction. In all other time intervals, the channel capacity is available for further, individual voice connections. It can be seen, that the fact according to which a traditional speech connection according to the conditions described above is only required from time-to-time during a conversation is advantageously exploited by means of such a method for improved exploitation of the channels. The existing transmission capacity of a line group can be exploited far better with speech interpolation. It is possible, for example, to load a group consisting of 35 or more lines with approximately twice as many calls as is conventionally possible.
Given connections having paths which are through-connected in time sections for a short speech phase, one refers to so-called virtual connections. According to standard methods, speech interpolation always requires a brief intermediate storage of digital signals which are formed and represent so-called speech phases or phrases. Therefore, it must be seen to that the upper limit of the transmission time from subscriber-to-subscriber, which is fixed at 400 ms, is not exceeded. If one takes into consideration that a satellite section already uses 300 ms transmission time, then only approximately 100 ms time remains for the other sections. This fact is of special significance, particularly for world-wide speech connections.
The known methods of speech interpolation are fundamentally employed section-wise, for example, between two switching centers. For this purpose, each of the two switching centers has a group of outgoing channels available from which it allots the channels to the message transmitters, as required. In a complete telephone network, in contrast thereto, a star-shaped, or even a meshed network, usually exists and a connection usually comprises a number of sections. Because of the required intermediate storage time per section, however, far too great a transit time would occur given a plurality of sections (up to 12). This situation would therefore not be able to lead to a realistic solution.